This application is based on and claims priority under 35 U.S.C. xc2xa7 119 with respect to Swedish Application No. 9904061-0 filed on Nov. 10, 2000, the entire content of which is incorporated herein by reference.
The present invention generally relates to a tool holding assembly. More particularly, the present invention pertains to a device in a tool holding assembly for axially moving a rotatable shaft that carries a work performing device at one end of the shaft.
It is advantageous for quality reasons to construct machines for grinding holes so that the grinding wheel during rotation work can also move back and forth in the axial direction. The axial movability of the grinding wheel improves the quality of the holes, i.e., the degree of surface fineness and the straightness of the holes, as compared to a non-oscillating shaft or arrangement. The wear of the grinding wheel is also more uniform and less dressing is needed.
In known machines of this type, the arrangement is such that the entire headstock with the spindle and the grinding wheel must move axially in order to displace the rotating shaft in its axial direction. As the object is to achieve very rapid and short axial movements, these known types of machines are rather unsatisfactory. That is because the entire mass of the headstock, the spindle and the grinding wheel must be displaced rapidly, which requires a very stiff and clearance-free bearing arrangement as well as a powerfull driving motor. Also, the wear on the headstock and driving mechanism is high which means that a relatively lot of maintenance is typically necessary.
In manufacturing processes today, the speed of production is high and the speed of rotation is often well above 100,000 r/min. This means that in known types of machines, the rotatable shaft cannot move or oscillate axially at a satisfactorily high speed because the higher the speed of production, the higher speed of the axial motion that is necessary to achieve high quality in the performed work.
The big mass that must be moved in known machines is in the size range of 50-100 kg. This mass can cause vibration and limit the speed of oscillation and thereby the speed of production.
There has thus existed for a relatively long time a high commercial demand for a satisfactory solution to the above-described problems. While the problems mentioned above have been described in connection with grinding wheel, the same problems also exist in connection with other machineries with a rotatable shaft that carries a mechanism for performing work during rotation of the shaft. One such example is a drilling machine intended to perfom very small and fast axial movements, such as for use in manufacturing circuits cards.
German Patent Publication No. DE 31 23 199 A1 describes a construction for axially oscillating a rotating shaft on which a working tool, such as a grinding wheel, is arranged. The oscillation is achieved with the aid of two springs arranged at the opposite sides of a disk positioned on the shaft. The springs act against each other and are brought into sympathetic vibration for oscillation of the shaft in the axial direction. The device described in this German publication suffers from several drawbacks and disadvantages. The mass that is brought into oscillation is rather large which, as mentioned above, is a rather serious problem. Another significant disadvantage is that the speed of oscillation is restricted to the resonance frequency of the spring system.
Another attempted solution is described in Japanese Patent Publication No. 1-240266. This document describes a construction involving a rotatable shaft provided with a radially extending rotor and an electromagnet arranged on each side of said rotor. The shaft is oscillated axially by controlling the magnitude of the current to each one of the electromagnets and thereby the magnetic forces on the rotor. One drawback with this construction is that the rotor arranged on the shaft is rather heavy and this makes the rotating axis even heavier, which restricts speed of rotation of the shaft. Another drawback is that this construction, requiring the use of two electromagnets, is rather expensive. A further significant difficulty with the construction according to this Japanese publication is that the electromagnets and the rotor require a lot of space.
The device in accordance with the present invention is adapted to be used in a tool holding assembly for axially moving a rotatable shaft on which is arranged, at a first end of the shaft, a device for performing work during rotation of the shaft. The shaft possess a free second end, and an electro-magnetic mechanism is provided to affect the second end of the shaft and draw the shaft in the axial direction from the first end to the second end against the affect of pressure acting against the second end in the opposite axial direction. A device controls the electromagnetic mechanism to achieve the axial movement of the shaft during rotation of the shaft.
According to one form of the invention, the electromagnetic mechanism includes a journal arranged with its free end adjacent the second end of the shaft and a magnetic coil arranged around the journal for generating a magnetic field in the axial direction of the journal. The journal, magnetic coil and an end part of the shaft adjacent the second end are encased in a housing arranged to guide the magnetic field in a closed loop that includes the end part of the shaft, the journal and a gap between the second end of the shaft and the free end of the journal, whereby the magnetic field acts on the end part of the shaft with a force in a direction against the free end of the journal.
In the device in accordance with the present invention, just the rotatable shaft is moved or oscillated in the axial direction. The small mass that moves or oscillates increases the possibility for high accelerations and an optimized pattern of movement. The axial movement of the shaft is thus not restricted to a sinus form.